Most modern petrol and diesel engines employ high pressure direct fuel injection systems for optimum control of the fuel flow. This type of system places the injector inside the combustion chamber where it is directly exposed to the temperature and pressure of combustion. Deposits form when small amounts of fuel remain on the injector tip surface after the engine is stopped. The residual heat in the injector effectively cooks the fuel leading to a carbon-based deposit forming on the injector surface.

A second source of deposits originates from micro-contaminants in the fuel itself. The modern supply chain of fuel is complex and involves refining, sea freight, pipelines, tanker trucks and retail dispensing. Micro-contaminants form due the additives that may be introduced during the refining process, salt contamination during shipping and insufficient filtration in the vehicle itself. 

Both of these sources of deposits can lead to blocked nozzles in the injector tip and binding of the internal injector mechanism which can affect the flow of fuel into the combustion chamber. At best, the fuel flow may be restricted and potential uneven spray pattern which leads to a loss of power. At worst, the injector may work intermittently or will severely restrict the flow of fuel which can lead to a significant loss of power.